(1) Field of the Invention
The present invention relates to a backlight unit and a liquid crystal display device using the same. More particularly, the present invention relates to a backlight unit using a light-emitting diode (LED) light source suitable for an application for uniformly reproducing an image on a panel, and a liquid crystal display device using the same.
(2) Description of the Related Art
Conventionally, a fluorescence lamp such as a cold cathode fluorescent lamp (CCFL) and an external electrode fluorescent lamp (EEFL) have been used as a light source of a backlight unit which irradiates a liquid crystal display (LCD) device.
However, recent years have seen a trend that a light-emitting diode (LED) is used as a light source of a backlight unit of a liquid crystal display device. The LED is a semiconductor element that emits light when a forward voltage is applied. The LED provides a longer operating life and a simpler structure than conventional light emitting elements (for example, CCFL, EEFL) materials, thus allowing mass-production in low cost. Further, the LED also provides low power consumption and favorable color reproducibility.
Common backlight units are classified into two types: the direct type which arranges a light source under a liquid crystal panel, and the edge light type which arranges a light source on a lateral side of a liquid crystal panel. JP-A-2007-293339 discusses an edge light type backlight unit having a LED light source.
The backlight unit discussed in JP-A-2007-293339 is an edge light type back light unit in which the light guide plate guides the light of the LED light source coming from a lateral side thereof to a liquid crystal panel. The light guide plate is, for example, an acrylic plate made of transparent resin having a surface specially processed for uniform planar emission of the light coming from an end face. This light guide plate enables local luminance control in response to an image signal to provide a uniform front luminance on the top surface in an area thereon.
A structure of a light guide plate and an optical distribution thereon will be briefly described below with reference to FIGS. 15A to 15C and 16.
FIGS. 15A to 15C illustrate a state in which one area of a light guide plate 10 emits light.
FIG. 16 is a graph illustrating a front luminance above the light guide plate when the one area of the light guide plate 10 emits light.
FIG. 15A is a perspective view of the light guide plate 10 seen from the top. Of areas of the light guide plate 10, only an area 10a emits light and other areas 10b do not. FIG. 15B is also a perspective view of the light guide plate 10 seen from another angle, illustrating a plurality of areas arranged in matrix form. FIG. 15C illustrates a state in which the area 10a of the light guide plate 10 emits light seen from a liquid crystal panel.
Thus, the light guide plate 10 is partitioned in areas so that the top surface (a surface facing the liquid crystal panel) of each area has a uniform luminance. Actually, the front luminance above the light guide plate 10 taken along the A-A′ line of FIG. 15A is illustrated in FIG. 16.
With the light guide plate 10 which is partitioned in areas as illustrated in FIG. 15, since each area is affected by adjacent areas, a method for controlling a light-emitting state of one area and light-emitting states of other areas is a matter of importance. One possible solution for this problem is to control an area to emit light after detecting information about light-emitting states of other areas. With this method, however, since it is necessary to know and control a light-emitting state of surrounding areas in order to allow one area to emit light, an image control algorithm becomes complicated causing a problem of an increase in circuit scale.
Further, when using a light guide plate which is partitioned in areas, there is a problem that an image looks discontinuous at an area boundary. Specifically, if the light does not uniformly change at the area boundary, a problem that image discontinuity becomes noticeable arises.
Meanwhile, there is a trade-off relation between the independency of a light-emitting state of a certain area from light-emitting states of other areas and a uniform change in light-emitting state at an area boundary. For ideal area control, it is necessary to maintain a uniform front luminance above a target area which is emitting light and, while maintaining the independency of a light-emitting state for each area to a certain extent, cause leakage of a certain amount of light such that the light-emitting state uniformly changes at the area boundary.
The present invention was devised in order to solve the above-mentioned problem. The present invention is directed to providing a backlight unit having a LED light source and utilizing an area-partitioned light guide plate which provides an ideal front luminance distribution.